Swellable polymeric controlled release drug delivery devices often suffer from the disadvantage that unexpected or variant conditions in the environment of use, for example levels of acidity, can predominate the rate or pattern with which the device releases its drug or active agent. For example, the rate of swelling (and hence drug release) of a pH sensitive, swellable, polymeric drug delivery device introduced into the gastrointestinal tract will vary depending on the acidity of the particular part of the tract through which the device passes. Such variation defeats the desired objective of drug release that is controlled by the dosage form rather than the environment and can, in fact, be detrimental to the patient. Overdosing or underdosing could occur if, at any one site (or within any given patient), conditions are such that an unwanted degree of polymeric swelling or deswelling causes substantially more or less of the drug to be released.
Although swellable, polymeric drug delivery devices are well known and are described, e.g., in U.S. Pat. Nos. 4,839,177; 4,747,847; 4,723,957; 4,350,271; 4,309,996; and 4,642,233; known polymeric devices fail to solve the aforementioned problems attendant to the use of polymeric drug delivery devices in environments of use whose conditions vary from location to location. Additionally, provision for control of polymer swelling and attendant beneficial agent release in variable or non-variant environments have not been disclosed previously.